Microfiltration (MF), Reverse Osmosis (RO) and Ultrafiltration (UF) membranes are often made from polymers, which swell and dissolve in organic solvents. Such polymeric membranes may dissolve in different solvents, but generally the best solvents for dissolving the polymers are dimethylformamide (DMF), N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), hexamethyl phosphoramide, sulfolane (tetramethylene sulfone) and N,N-dimethylacetamide.
There are many potential membrane applications which could usefully employ solvent stable membranes. Such applications are e.g. in the areas of food technology, biotechnology, the treatment of waste streams, chemical manufacturing and petrochemicals. These solvent stable membranes are desirably also swelling resistant, because swelling of such membranes under pressure would indicate solvent/membrane interaction and thus polymer chain mobility, which usually results in compaction, and loss of flux and of performance generally, under pressure.
A particular advantage of such solvent stable membranes would be, that they could be exposed to a variety of solvent media including aqueous solutions, suspensions or emulsions, as well as to organic solvents which contain solutes.
There are presently available solvent stable membranes in the form of ceramics or other inorganic materials and specialized crosslinked polymers. These, however, are expensive, limited in cutoff range to MF and UF, and restricted in practice to certain configurations. Besides ceramics, there are available membranes from crosslinked polymers such as epoxy polyimide type polymers, as well as encapsulated polymers. Encapsulated polymers are described in U.S. Pat. No. 4,778,596; the membranes are coated on the external surfaces and on the internal pore surfaces with an aqueous polymer which is then crosslinked. The support membrane is not crosslinked, but encapsulated in an outer skin; such membranes swell but do not dissolve, upon immersion in organic solvents. Crosslinked polyimides commercialized by NITTO (Japan) have some solvent resistance but limited pH/temperature stability; in addition, the membranes are limited to the UF range with low efficiency in operation.